Generally, when modulating an LD at a high speed, pulse width of optical output waveform is narrower compared to that of an LD drive current waveform, due to turn-on delay in the LD. In particular, when non-bias modulating an LD, since there may be a turn-on delay time of about 1 nsec or more, depending on the type of LD, with a rise in bit rate, such a value cannot be ignored with respect to mask registration. This turn-on delay time Td can typically be expressed by the following equation (1), provided that a carrier lifespan is τs, an LD drive current is If, a bias current is Ib, and an LD threshold current is Ith.Td=τs×In[(If−Ib)/(If−Ith)]  (1)
From the above equation (1), it can be understood that the turn-on delay time Td fluctuates according to the LD drive current If, the bias current Ib, and the LD threshold current Ith. Also, in the turn-on delay time Td, there is an individual difference due to connection variation in the assembly of the LD optical system. Further, even if an LD is decided, since there is a change in the turn-on delay time Td due to temperature fluctuation, pulse width fluctuation in an LD light output waveform is caused. Consequently, a control circuit is required for the purpose of suppressing pulse width fluctuation in the LD light output waveform due to temperature fluctuation and the like.
FIG. 13 is a circuit diagram showing a structural example of a conventional pulse width control circuit.
In FIG. 13, the conventional pulse width control circuit comprises a Tr/Tf control section that controls a rise time Tr and a fall time Tf of an input data signal DATA_IN, and a waveform shaping section that waveform shapes a signal from the Tr/Tf control section to output an output signal DATA_OUT for driving an LD.
In this conventional circuit, as shown in FIG. 14, in the Tr/Tf control section, the rise time Tr of the input signal DATA_IN is restricted by a time constant determined by a constant current source I11 and capacity C11, and the fall time Tf of the input signal DATA_IN is restricted by a time constant determined by a constant current source I12 and the capacity C11 (refer to a voltage waveform at point BB). Then, in the waveform shaping section, the output signal from the Tr/Tf control section and a threshold voltage (refer to a voltage waveform at point CC) set according to pulse width control information supplied from the exterior are compared with each other, so that the output signal DATA_OUT whose pulse width is controlled is generated to be output. Here, as shown on the left side of FIG. 14, the threshold voltage is set to be high, so that the output signal DATA_OUT with a narrow pulse width is output. As shown on the right side of FIG. 14, the threshold voltage is set to be low, so that the output signal DATA_OUT with a broad pulse width is output.
An LD drive circuit is connected to a latter stage of the conventional pulse width control circuit shown in FIG. 13. In the LD drive circuit, an LD drive current is controlled in accordance with the output signal DATA_OUT of the pulse width control circuit. As a result, pulse width control information that brings pulse width of an LD light output waveform to a desired value is sent to the pulse width control circuit, and the threshold voltage is set according to the pulse width control information to set pulse width of an optical output.
However, in the conventional pulse width control circuit as described above, the input signal DATA_IN is band restricted by the Tr/Tf control section so that the rise and fall time constants are always constant, and then, compared with the threshold value controllable from the exterior, so that the pulse width of the output signal DATA_OUT is controlled. Therefore, there is a problem that it is difficult to respond to multi-bit rate of data signals.
In other words, as shown on the left side of FIG. 15 for example, in the case where the time constant of the Tr/Tf control section is set to correspond to a bit rate f, if this is made to correspond to a four times bit rate 4f, the rise time is changed due to the state of previous bits, as shown in the voltage waveform at point BB in the lower part of the figure, and a pattern effect is generated in the output signal DATA_OUT. In the example of the figure, there occurs a pattern effect such that each pulse rise delay time becomes t1=t2<t3. On the other hand, as shown on the right of FIG. 15, in the case where the time constant of the Tr/Tf control section is set to correspond to the fast bit rate 4f, if this is made to correspond to the bit rate f, an adjustment range of the pulse width controllable according to the setting of threshold voltage becomes narrower, causing a possibility that a desired pulse width cannot be realized.
Also, in the conventional pulse width control circuit, there is a disadvantage that it is easily affected by noise or power source voltage fluctuation. In other words, as shown in FIG. 16 for example, in the case where the pulse width is set to be wide, it is necessary to set the threshold voltage of a comparator of the waveform shaping section to be low. Consequently, the waveform shaping section results in a circuit that is liable to be affected by ground (GND) noise and the like, to easily cause pulse width fluctuation or the pattern jitters. Note, if polarity of the circuit shown in FIG. 13 is reversed, the waveform shaping section results in a circuit that is liable to be affected by power source noise. In addition, the Tr/Tf control section has a circuit structure that is liable to be affected by power source noise or ground noise, irrespective of a set value of the pulse width, since it operates in a state where the band thereof is always lowered.
Further, in the conventional pulse width control circuit, in order to set the pulse width of the output signal DATA_OUT to be wider, 100% or more, than the pulse width of the input signal DATA_IN, it is necessary to restrict the band of the fall time Tf for the input signal DATA_IN. However, as shown in FIG. 17 for example, if power source voltage fluctuation occurs, there is a difference in the time taken for the voltage at point BB to change from a low level to a high level and to reach the low level again, and since this signal is compared with the direct current threshold voltage in the waveform shaping section at the latter stage, as a result, the pulse width fluctuation is caused by the power source voltage fluctuation. Also, in the case where the polarity of the circuit shown in FIG. 13 is reversed, there is caused the same problem as in the case described above when the pulse width is set to be narrow.
The present invention has been accomplished in view of the above problems, and has an object to provide a pulse width control circuit capable of responding to multi-bit rates with the same circuit structure. Also, the present invention has a further object to provide a pulse width control circuit that has excellent noise resistance and is not liable to be affected by power source voltage fluctuation.